The polymerization of olefins in two or more serially connected gas-phase reactors allows for the production of olefin polymers with improved properties and/or simplification of the existing production processes. This process is made possible by choosing polymerization conditions in the second reactor or subsequent reactors that differ from the reaction conditions existing in the first polymerization reactor. Olefin polymers may grow on particles that include a catalyst component, which continues to exert a catalytic activity even when the polymer particles are transferred to a successive gas-phase reactor. The polymer resulting from the first gas-phase reactor may be transferred to a second gas-phase reactor, where polymerization is continued under different conditions. Therefore, different fractions of polymer can grow on the same particle by maintaining a different composition of the gas-phase mixture in each reactor.
Examples of polymers that may be produced by a multistage gas-phase process include bimodal or multimodal polymers obtained by maintaining a different concentration of a chain terminator, such as hydrogen, in each reactor; and random or heterophasic copolymers obtained by polymerizing different (co)monomers in each reactor. The term “heterophasic copolymer” includes in-reactor polymer blends.
The transfer of the polymer from one gas-phase reactor to another one is a further step in a multistage polymerization process. A direct discharge of polymer from an upstream reactor to a downstream reactor does not allow for maintaining significantly different polymerization conditions in the downstream reactor due to the amount of gases and dissolved hydrocarbons associated with the polymer transferred to the downstream reactor.
A possible solution to this problem is degassing the solid polymer discharged from the upstream reactor, then subjecting the polymer to a compression stage and transferring it to the downstream polymerization reactor. A similar process is disclosed in EP 192 427 A1, which describes a process in which the compression stage is performed by means of the reaction gas mixture of the downstream reactor at a temperature lower by at least 20° C. than the temperature of the downstream reactor. EP 050 013 A2 refers to a process for polymerizing an olefin in the gaseous phase in a multiplicity of steps in at least two independent polymerization zones connected to each other by a transfer passage by which a gaseous stream containing the polymer obtained in a first polymerization zone is transferred into a second polymerization zone. The process is characterized in that an inert gas zone is provided in the transfer passage and at least a part of the gas components of the gaseous stream containing the polymer is replaced by an inert gas. However, the individual steps of the transfer processes described therein must be carried out subsequently, such that the operations are performed periodically and therefore do not provide a continuous transfer of polymer from the upstream reactor to the downstream polymerization reactor.
EP 1 040 868 A2 discloses a method of multistage gas phase polymerization in which polymerization of a feed gas mixture comprising ethylene, an alpha-olefin and hydrogen is carried out in an upstream arranged fluidized-bed reactor. The polymer powder taken up from the upstream arranged fluidized-bed reactor is treated with a gas to lower the content of alpha-olefin gas and hydrogen gas in the polymeric powder and then introduced into a downstream arranged reactor. WO 2008/058839 A2 discloses a process for the multistage polymerization of olefins which allows for continuously discharging of the polymer and the gas reaction mixture from the upstream reactor into a transfer device and continuously feeding of the polymer from the transfer device to a downstream reactor via a transfer device comprising a separation chamber, in which the gas reaction mixture is removed from the polymer, and at least two lock hoppers, which work intermittently in parallel. A
Although both processes reduce the amount of gas components of the reaction gas of the upstream reactor, which are transferred along with the polyolefin particle to the downstream reactor, minor amounts of the reaction gas components of the upstream reactor are still carried over into the downstream reactor. A reliable process for continuously transferring polyolefin particles from a first gas-phase polymerization reactor to a second gas-phase polymerization reactor which allows little or no reaction gas of the first gas-phase polymerization reactor to be introduced into the second gas-phase polymerization reactor would therefore benefit olefin polymerization and polymer processing.